The invention relates to vertical-cavity surface emitting lasers (VCSELs) and VCSEL-based devices, and more particularly to polarization control of VCSELs.
High density semiconductor laser arrays are promising for light sources of optical communication and optical computer systems.
In a manufacturing processes of a laser array, there is a flip chip bonding process, wherein separately produced laser elements are aligned on a common heat sink and are adhered to the surface of the heat sink by such low melting point metals as solder or AuSn. As for the alignment of the elements, a mechanical process is employed, and it is difficult to obtain a high density and high precision of alignment.
As another manufacturing process of a laser array, there is a monolithic process, wherein an array of laser elements is produced on a single substrate. In an edge emitting laser array as shown in FIG. 29, an array of mesa-stripes 106 is produced by monolithic process on a substrate. Each laser element including an active layer 107 can be driven separately by power-supplying through the corresponding electrodes 104. Alignment precision of this laser array is sufficiently high, since the alignment is determined by optical exposure.
A disadvantage of the edge emitting laser array shown in FIG. 29 is that the array is one-dimensional and is not good for use as a light source of an information processor.
An array of surface emitting lasers is more promising because it is two-dimensional, and high density alignment is easy, and coupling coefficient for coupling an array of surface emitting lasers to optical fibers is high. In optical switching or in optical interconnection, a laser array of surface emitting lasers of high density and high alignment accuracy is expected.
However, polarization control of a surface emitting laser is more difficult than that of an edge emitting laser.
FIG. 30 shows a cross-sectional view of a VCSEL of the prior art. An active layer 3 of InGaAs and optical confinement layers 4 of AlGaAs (the layers 3 and 4 are called intermediate layers) are sandwiched between a p-side semiconductor multi-layer reflector 2 (or p-side Distributed Bragg Reflector, and hereafter called a p-DBR 2) and an n-side semiconductor multi-layer reflector 6 (hereafter called an n-DBR 6). In an embodiment, the p-DBR 2 has 15.5 pairs of GaAs/AlAs layers, and the n-DBR 6 has 18.5 pairs of GaAs/AlAs layers. A resonator cavity is constituted by the p-DBR 2 and the n-DBR 6. The p-DBR 2 is formed into a post by etching, and the active layer 3 is inactivated by ion implantation leaving a region just underneath the post-type p-DBR 2 as shown by an inactivated region 5 in FIG. 30. A current flowing between an anode 1 and a cathode 7 excites laser oscillation, and generated laser light is emitted through a plane 100 of the substrate.
The plane 100 is called a plane of laser light emission, and each layer of the intermediate layers and the semiconductor multi-layer reflectors is formed on the substrate in parallel to the plane of laser light emission 100.
A VCSEL of FIG. 30 has a narrower beam angle and a larger mode separation than an edge emitting laser, and is more adapted to compose a laser array.
As is apparent from FIG. 29, polarization of emitted light of the edge emitting laser array is not influenced by array configuration as long as the pitch of the element arrangement is large enough to avoid mode coupling between neighboring elements. Thus, polarization of emitted light of a laser array of edge emitting lasers is determined by polarization of an element, wherein differences of waveguide loss and reflection coefficient between TE (transverse electric) and TM (transverse magnetic) modes are determined by the shape of the mesa stripe 106. In an embodiment shown by FIG. 29, polarization generated by TE mode in parallel to the substrate is predominant.
In order to obtain a polarization in a desired plane, the substrate of an edge emitting lasers must be held in parallel to the desired plane. Therefore, it will be very difficult to hold edge emitting lasers of different polarization on a common heat sink.
On the other hand, a VCSEL shown in FIG. 30, has no factor for influencing the polarity of the emitted light, and therefore, the polarization of the emitted light from an element laser will be a random and unstable, discontinuous change of polarization (called switching) often arising from temperature change or from current intensity change.
In recent applications of lasers, it is usual that such polarization-sensitive devices as beam splitters or polarizers are usually combined with the lasers, requiring a precise and stable polarization of emitted laser light. Even when polarization-sensitive devices are not used in combination with a laser, stability of polarization is still required, because change of polarization may cause change of reflection at end surfaces of devices, and the change of reflection may cause instability of a system.
Thus, polarization control of a VCSEL has been a problem in optical communication systems and in optical computer systems, and various proposals and attempts have been reported in this technical field. In one of such attempts, there is "A Method of Polarization Stabilization in Surface Emitting Lasers" by Mitsuaki Shimizu et al reported in Japanese Journal of Applied Physics vol. 30 pp. L1015-L1017, 1991. In this attempt, anisotropy of reflection coefficient is introduced for controlling polarization. Two side planes facing each other of the post type p-DBR 2 are coated by metal of high reflection coefficient. The validity of this attempt is not confirmed by experiments.
Another attempt is reported in "Polarization Control of Surface Emitting Lasers by Anisotropic Biaxial Strain" by Toshikazu Mukaihara et al in Japanese Journal of Applied Physics vol. 31 pp. 1389-1390, 1992. In this attempt, an elliptical groove is formed in the substrate, producing anisotropic strain in the path of the laser light. This anisotropic strain causes anisotropic reflection coefficient and polarization parallel to the major axis of the ellipsis becomes predominant. But there may arise various kinds of noisy strains from thermal expansion or from packaging stress.
In a Japanese patent application entitled "A semiconductor laser" and laid open as a Provisional Publication No. 265584/'89, a rectangular waveguide having a high refraction coefficient is provided in the laser path. A polarization in parallel to the longer sides of the rectangular waveguide is expected. But effectiveness of optical confinement in the high refraction waveguide is doubtful. When the laser light is not effectively confined in the waveguide, it will be difficult to obtain polarized light of sufficient intensity.
In a Japanese patent application entitled "A semiconductor laser" and laid open as a Provisional Publication No. 242989/'92, electrodes of a laser element are shaped in anisotropic form in a gain confinement type laser for obtaining anisotropic gain. But in a gain confinement type laser, the optical confinement is weak and the laser light will be diffused. Because of this diffusion, the ratio of light intensity confined underneath the electrodes (the coefficient of the optical confinement) is very small. Therefore, anisotropic gain obtained by anisotropic shape of electrodes will be small, and will give only a weak polarization effect.
In a Japanese patent application entitled "A surface emitting semiconductor laser" and laid open as a Provisional Publication No. 144183/'92, anisotropy of reflection coefficient is introduced by the anisotropic cross-sectional shape of the resonator cavity.
For example, the shapes of the cross-section of the resonator cavity of this prior art are as shown in FIG. 10. In this prior art, the reason why this anisotropic cross-section can control polarization is not described. In IEEE Photonics Technology Letters vol. 6, pp. 40-42, 1994, it is described that a rhombus 109 of FIG. 10 is ineffective in polarization control. In the aforementioned Japanese patent application entitled "A semiconductor laser" and laid open as a Provisional Publication No. 265584/'89, it is described as a prior art that an ellipse 108 of FIG. 10 is ineffective in polarization control. From these descriptions, it will be understood that the anisotropic cross-section cannot control polarization when there is no other factor for controlling polarization.